A typical optical transceiver module currently used in optical communications includes a transmitter portion and a receiver portion. The transmitter (TX) portion includes a laser driver, which is typically an integrated circuit (IC), one or more laser diodes, and an optics system. The laser driver outputs electrical signals to the laser diodes to modulate them. When the laser diodes are modulated, they output optical signals, which are then directed by the optics system of the TX portion onto the ends of respective transmit optical fibers or waveguides held within a connector that mates with the transceiver module. The TX portion typically also includes an open loop or closed loop optical output power control system for maintaining the average optical output power levels of the lasers at substantially constant levels.
The receiver (RX) portion of the optical transceiver module typically includes at least one photodiode, at least one TIA, and at least one limiting amplifier (LA). The photodetector, which is typically a P-intrinsic-N (PIN) photodiode, produces an electrical current signal in response to light detected by the photodetector. The TIA forms the front-end of the RX portion. The photodiode converts the input light into an electrical current signal and presents it at the input of the TIA. The TIA converts this electrical current signal into an output voltage having some gain, commonly referred to as transimpedance gain, and this signal is further processed by other stages (i.e., the LA, output driver, etc.) in the RX portion.
The TIA handles input signals (the photodiode output) of varying optical modulation amplitude (OMA) and average power (Pavg), and therefore needs to have a wide input dynamic range. OMA is expressed as OMA=P1−P0, where P1 is the optical power generated by the laser diode when it is in the logic 1 state and P0 is the optical power generated by the laser diode when it is in the logic 0 state. The average optical power is expressed as Pavg=(P1+P0)/2. Another important term is extinction ratio (ER), which is defined as the ratio between the two optical power levels, ER=P1/P0. OMA is related to Pavg and ER as: OMA=2*Pavg*(ER−1)/(ER+1). The photodector creates an average current, IAVG, corresponding to PAVG and has a current amplitude, CA, corresponding to OMA. The TIA amplifies the current to create a modulated signal with a Voltage Modulation Amplitude, VMA, which is used in some cases to control the automatic gain control function.
Wide input dynamic range calls for the use of an automatic gain control (AGC) circuit in the RX portion for automatically adjusting the gain of the TIA based on the amplitude of the input signal. Without an AGC circuit, the TIA tries to convert the current into a corresponding output voltage with its transimpedance gain as the amplitude of input signal current increases. When this happens, however, the transimpedance gain is limited by the voltage headroom (the maximum high and low output voltage for linear operation of the TIA) as the output voltage swing increases, which results in the output signal becoming distorted. Hence, an AGC circuit is needed in order to lower the gain of the TIA as the amplitude of the input signal grows so as to prevent the TIA from saturating and producing distortion at its output. In addition, the TIA also needs to operate at multiple data-rates, which requires adjustment of the bandwidth of the TIA.
FIG. 1 is a block diagram of a typical TIA circuit 2 that has resistive feedback architecture. The TIA circuit 2 comprises a feedback resistor RF 3, a first metal oxide semiconductor field effect transistor (MOSFET) MBW 4, a second MOSFET MDC 5, a resistor 6, a third MOSFET MAGC 7, a photodiode 8, first and second bipolar junction transistors (BJTs) 9 and 11, a current source 12, and a dummy side 13. The dummy side 13 comprises resistors 14 and 15, BJTs 16 and 17, and current source 18 that mirror resistors 3 and 6, BJTs 9 and 11, and current source 12, respectively. The value of the feedback resistor, RF, 3 is either fixed or minimally adjustable and serves to set the gain and bandwidth ranges of the TIA circuit 2. The value of RF 3 varies quite a bit (e.g., 25 to 30%) over process and temperature. Some of the process variations can be calibrated out, but temperature variations will continue to affect the gain and bandwidth of the TIA circuit 2. The bandwidth voltage, VBW, which is applied to the gate of MBW 4, is a digitally-controlled signal that changes the effective bandwidth of the TIA circuit 2 by turning MBW 4 ON and OFF. When MBW 4 is turned ON, the TIA circuit 2 operates at a first data rate having a first bandwidth. When MBW 4 is turned OFF, the TIA circuit 2 operates at a second data rate having a second bandwidth.
The operation of MDC 5 is controlled by a direct current (DC) cancellation signal, DCCAN, which is driven by a DC cancellation block (not shown for purposes of clarity). MDC 5 is operated in a manner that causes TIAOUT<0> and TIAOUT<1> to track one another by sinking the average input current through MDC 5. A replica of this average current is pushed into a fixed resistor to generate the AGC voltage signal, VAGC, which turns ON MAGC 7. MAGC 7 turns ON stronger as the average input current increases and hence reduces the effective feedback resistance RF 3 and the gain of the TIA circuit 2 to allow it to handle a larger signal at the input where the TIA circuit 2 connects to a photodiode 8.
One of the disadvantages of the TIA circuit 2 and similar designs is that they have a limited dynamic range, and therefore are not capable of adequately handling input signals of varying OMA and Pavg. Another disadvantage of such TIA circuits is that while some of the process variations associated with variations in the value of RF can be removed through calibration, temperature variations that cause the value of RF to vary generally cannot be removed. Therefore, the performance of such TIA circuits can be detrimentally affected by temperature variations. Yet another disadvantage of such designs results from the operation of MAGC 7. The signal VAGC that controls the operations of MAGC 7 is an analog signal. When the value of VAGC is such that MAGC 7 is not turned fully ON or fully OFF, MAGC 7 operates in a nonlinear region, which can result in distortion in the output of the TIA circuit.
Accordingly, a need exists for a TIA circuit that has a wide dynamic range over multiple data rates, that has performance characteristics that are independent of temperature variations, and that avoids the aforementioned problems that can lead to distortion in the output of the TIA circuit.